In physiology, electrotonus refers to the "passive" spread of charge inside a neuron. "Passive" means that voltage-dependent changes in membrane conductance do not contribute. Neurons and other excitable cells produce two types of electrical potential. The first is a non-propagated local potential called an electrotonic potential, due to a local change in ionic conductance (e.g. synaptic or sensory that engenders a local current). When it spreads along a stretch of membrane, it becomes exponentially smaller (decrement). The second is a propagated impulse called an action potential. Electrotonic potentials represent changes to the neuron's membrane potential that do not lead to the generation of new current by action potentials. Neurons which are small in relation to their length, such as some neurons in the brain have only electrotonic potentials (starbust amacrine cells in the retina are believed to have these properties); longer neurons utilize electrotonic potentials to trigger the action potential.
The electrotonic potential travels via electrotonic spread, which amounts to attraction of opposite- and reflection of like-charged ions within the cell. Electrotonic potentials can sum spatially or temporally. Spatial summation is the combination of multiple sources of ion influx (multiple channels within a dendrite, or channels within multiple dendrites), where temporal summation is a gradual increase in overall charge due to repeated influxes in the same location. Because the ionic charge enters in one location and dissipates to others, losing intensity as it spreads, electrotonic spread is a graded response. It is important to contrast this with the all-or-none law propagation of the action potential down the axon of the neuron.
Electrotonic spread is generally responsible for increasing the voltage of the soma (neuronal cell body) sufficiently to exceed threshold and trigger the action potential; its summation properties described above make it suitable for integrating input from many different sources. Such input may be depolarizing (positive charge, such as sodium) or hyperpolarizing (negative charge, such as chloride).
Electrotonic potentials are conducted faster than action potentials, but attenuate rapidly so are unsuitable for long-distance signaling.
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